Leaching is a cost effective method for recovering precious metals such as gold or silver from ores. In the process, a lixiviant system, comprising a ligant and an oxidant is used to dissolve out the desired precious metal from the ore. As used herein, the phrases “lixiviant system” and “lixiviant solution” will be used interchangeably and do not imply a true chemical solution—only a chemical combination adapted to extract the mineral value in the ore.
In heap leaching, the metal bearing ore may be obtained from an open pit mine or the like and is crushed to produce an aggregate that is coarse enough to expose the desired mineral values but fine enough to allow intimate contact of the lixiviant system or solution therewith. The lixiviant solution may be distributed over the top of the metal ore heap via sprinklers, wobblers, or other similar equipment. The barren lixiviant “percolates” through the heap to perform its desired function with the metal and the resulting “pregnant” solution is then collected by an impervious leach pad or the like located at the bottom of the heap. The pregnant solution is then subjected to conventional mineral recovery techniques to obtain the desired precious metal.
In gold heap mining operations, a lixiviant system comprising cyanide, air and lime is commonly used under highly alkaline conditions (pH 9–11.5) to form the pregnant solution, (i.e., a complex or ligand coordinated with a gold cation). The gold cation complex or ligand leaches from the ore heap and is recovered. The gold is then separated from the lixiviant complex via conventional separation techniques such as the conventional method of adsorption on an activated carbon column or bed.
It has been discovered that in some leach mining operations, the metal ore itself can rob or adsorb the metal value that is complexed with the lixiviant in the pregnant lixiviant solution. That is, the amount of metal contained within the pregnant lixiviant solution is depleted by the metal ore itself. This undesirable action is referred to as “preg-robbing” since the pregnant lixiviant solution is robbed or depleted of the desired solubilized metal. Although applicants are not to be bound to any scientific explanation as to the reason for this phenomenon, it is thought that the problem may be caused by presence of graphite or other carbonaceous matter in the raw metal ore.
Presently, preg-robbing is inhibited by the addition of kerosene, which is highly flammable. In other cases, the ore is roasted or treated in autoclaves at high temperatures. These latter two alternatives are energy intensive, resulting in overall high metal recovery costs.
It is therefore an object to provide an effective alternative to present day preg-robbing inhibition treatments and methods.